The grooves served 2 purposes. They allowed a better grip when bayonet fighting with cold, wet, muddy hands. They also allowed you a better grip when assuming or standing from prone with the same cold, wet, muddy hands.
Appearance seems utterly random, with the exception that I've never seen grooved blond wood. I've had a '43 with the grooves, a '42 without, & a blond '55 without.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”